(a) Field of the Invention
The present invention generally relates to a Radio Frequency (RF) filter. More particularly, the present invention relates to a dielectric resonator in an RF filter.
(b) Description of the Related Art
An RF filter (e.g. a Dielectric Resonator (DR) filter, a cavity filter, a waveguide filter, etc.) has a kind of circuit cylinder structure for resonating at a radio frequency or ultra radio frequency. A typical coil-condenser resonant circuit is not suitable for generating an ultra radio frequency due to a large radiation loss. The RF filter has a plurality of resonators each forming a metal cylindrical or rectangular cavity coated with a conductive material and a dielectric resonance element or a resonance element configured to be a metal resonance rod is provided in the cavity. The resulting existence of an electro-magnetic field only at a unique frequency makes ultra radio frequency resonance possible.
RF filters may be categorized into Transverse Magnetic (TM) mode, Transverse Electro Magnetic (TEM) mode, and Transverse Electric (TE) mode according to their resonator structures. An exemplary TM-mode resonator with excellent Quality factor (Q) characteristics is disclosed in U.S. Pat. No. 7,106,152 entitled “Dielectric Resonator, Dielectric Filter, and Method of Supporting Dielectric Resonance Element” by Takehiko Yamakawa, et. al. for which a patent was granted on Sep. 12, 2006.
Compared to a conventional TEM-mode resonator (a cavity filter structure), since a TM-mode resonator has a high Q value, it has Q characteristics improved by 40% for the same size. Owing to these characteristics, the TM-mode resonator filter can be designed to be much smaller, to have less insertion loss for the same size, and to have better attenuation characteristics than the TEM-mode resonator filter.
Although a TE01δ-mode resonator filter has a three times higher Q value than the TEM-mode resonator filter, it requires a few times higher fabrication cost and a huge volume. That's why the use of the TE01δ-mode resonator filter was restrictive to a Base Station (BS) high-power filter. Thus, the TE01δ-mode resonator filter is not feasible for small-size products.
FIG. 1 illustrates the structure of a conventional TM-mode resonator. Referring to FIG. 1, the conventional TM-mode resonator has a dielectric resonance element 5 at the center of a housing space defined by a metal cover 3 and a housing 4. Notably, both end surfaces of the dielectric resonance element 5 are brought into close contact with inner upper and lower surfaces of the housing space. A tuning groove may be formed at an upper end portion of the dielectric resonance element 5 and a tuning screw 1 and a fixing nut 2 are installed at a position corresponding to the tuning groove, for frequency tuning.
In this structure, it is very significant to assemble the dielectric resonance element 5 so that both end surfaces of the DR element 5 closely contact the inner upper and lower surfaces of the housing space. If the assembly is not done reliable, the characteristics of the TM-mode resonator are greatly changed with temperature changes, making it impossible to apply the TM-mode resonator to commercial products.
To avert this problem, metal coatings 6 are typically formed on both ends of the dielectric resonance element 5 and then the dielectric resonance element 5 is combined with the housing 4 and the cover 3 by soldering or an adhesive, or by any other method, as illustrated in FIG. 2.
The TM-mode resonator may be fabricated by use of a metal plate and other accessories instead of the metal coatings. However, it is difficult to assemble all dielectric resonance elements of the RF filter with the same force due to the processing tolerances of the dielectric resonance elements and the housing, thus making fabrication difficult. Especially since the dielectric resonance elements and the housing have different thermal expansion coefficients, the fixed or contact states of the dielectric resonance elements become poor and filter characteristics change, due to their contraction and expansion with temperature changes.